The overall goal of this research is to determine the function of the individual alpha subunit isoforms of the Na,K-ATPase and the role each plays in cardiac physiology. The Na,K-ATPase utilizes the energy derived from ATP hydrolysis to catalyze the transport of 3 Na+ ions out of the cell and 2 K+ ions into the cell. This enzyme produces electrical and chemical gradients which are necessary for a variety of cellular/physiological functions including the excitability of nerves and muscles and the restoration of calcium levels after cardiac muscle contraction. The Na,K-ATPase is a heteromeric enzyme composed of an alpha and a beta subunit. The alpha subunit, of which four isoforms exist, contains both the catalytic activity and is the receptor for cardiac glycosides. The cardiac glycoside class of compounds bind to and inhibit the Na,K-ATPase and are used in the treatment of congestive heart failure. Using genetic modulation of the Na,K-ATPase alpha isoforms, in knock-out and transgenic mice, we hope to identify specific functional roles for individual isoforms, better define the pharmacological aspects of cardiac glycoside therapy and provide a more detailed understanding of Na,K-ATPase inhibition as it relates to cardiac physiology.